Anti-static styrene polymer particles

ABSTRACT

Expandable styrene polymer particles which have anti-static and anti-lumping properties during expansion and after molding into an article are produced by adding 0.1 to 0.4 parts of trialkyl alkylalkoxylated quaternary ammonium salts, per 100 parts of polymer particles by coating of the expandable polymer particles with the salts. The salts may be added as sole antistatic agent or in addition to the known internal additives which impart fast-cool and anti-lump properties.

BACKGROUND OF THE INVENTION

This invention relates to a process for making expandable styrenicpolymers anti-static and anti-lumping during pre-expansion and aftermolding by coating certain quaternary ammonium salts onto the expandablepolymer particles or the molded articles.

The making of low density, cellular, shaped, plastic articles fromexpandable particles of styrene polymers is well known. Such particlesgenerally contain a blowing agent which boils below the softening pointof the polymer and which will cause the particles to expand when theyare heated. When the expanded particles are heated in a mold cavity, theparticles expand further to fill the mold and fuse together to form ashaped article.

The formation of molded articles from expandable styrene polymerparticles is effected in two steps: (1) pre-expanding of foamablestyrene polymer particles to a density of between 1 and 3 pounds percubic foot; and (2) further heating the pre-expanded particles in aclosed mold to cause further expansion and form a fused, one piecearticle having the shape of the mold. The second step is what isnormally referred to as "molding".

The pre-expansion step may be carried out by heating the expandablepolymer particles by any suitable heating medium such as steam, hot air,hot water, or radiant heat. A widely used method of pre-expansion of theparticles is a process such as that disclosed in U.S. Pat. No. 3,023,175by Rodman. Another method is that disclosed in U.S. Pat. No. 3,577,360by Immel, which teaches the pre-expansion of polymer beads by heating ina dry atmosphere followed by the application of a vacuum to cause thearticles to expand to the desired density.

An undesirable result of the pre-expansion before introduction into themold is the tendency of the pre-expanded particles to clump together andform lumps which render the particles unsuitable for molding. Theselumps cannot be properly conveyed in processing plants and areunsuitable for charging into molds of intricate contour due to improperfill-out of the molded articles. Even if reasonable fill-out isachieved, variations in density within the molded articles can resultand voids also occur.

Various methods have been proposed for the prevention of lumping duringpre-expansion, but these have all suffered from some objectionablefeature such as rapid loss of blowing agent, poor flow of the treatedbeads, dust problems which cause plugging of the steam ports, and oftenserious reduction in fusion of the particles on molding. U.S. Pat. No.3,520,833 teaches the addition of lecithin during the impregnation ofthe particles with the blowing agent. Unfortunately, the lecithinimparts an undesirable odor to the molded articles. U.S. Pat. No.3,462,293 teaches to coat the particles with polymeric materials by afluid bed process. This process involves an additional expense offluidizing the particles and coating with the polymer latexes. U.S. Pat.No. 3,444,104 teaches the addition of calcium silico aluminate. Thisadditive tends to foul transfer lines and molds with clay-like deposits.

Another problem encountered with styrenic polymer particles is that ofstatic charge on the particles, both before and after pre-expansion. Thestatic charges cause problems in conveying the particles smoothly into,for example, the pre-expander or during the molding operation. Anotherarea where static charges become a major problem is in the final moldedproduct. Thus, molded styrene polymer used to package electronicequipment can damage the enclosed equipment due to static discharge.Various anti-static agents have been used to attempt to overcome thestatic problems.

SUMMARY OF THE INVENTION

It has now been found that trialkyl alkylalkoxylated quaternary ammoniumsalts, when coated onto the surface of expandable styrene polymerparticles serve as an antistatic and anti-lumping agent for theparticles. Coating with from 0.10 to 0.40 parts of salt per 100 parts ofpolymer gives a foamable product which, when pre-expanded, exhibits lesslumping than expandable styrene polymers not having the quaternary saltspresent. Moldings made from the coated particles have better staticproperties than those molded from unmodified styrene polymers.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with this invention, a variety of expandable thermoplastichomopolymers and copolymers can be rendered anti-static andanti-lumping. The polymers may be derived from vinyl aromatic monomersincluding styrene, isopropylstyrene, alpha-methylstyrene, nuclearmethylstyrenes, chlorostyrene, tert-butylstyrene, etc., as well ascopolymers prepared by the copolymerization of a vinyl aromatic monomerwith monomers such as butadiene, alkyl methacrylates, alkyl acrylates,acrylonitrile and maleic anhydride wherein the vinyl aromatic monomer ispresent in at least 50% by weight of the copolymer. For the purpose ofconvenience, these polymers and copolymers are referred to herein asstyrene polymers.

Especially useful are polystyrene, and copolymers of styrene with 5-30weight precent of acrylonitrile, maleic anhydride, or methyl acidmaleate.

The styrene polymers may be in the form of beads, granules, or otherparticles convenient for the expansion and molding operations.Especially useful are the beads formed by the suspension polymerizationof the vinyl aryl monomers alone or in combination with the minor amountof copolymerizable monomers.

The styrene polymers, can, of course, be produced by any of the knowntechniques. The preferred method of preparation is the suspensiontechnique, which gives bead or pellet shaped product.

The styrene polymer particles may be made expandable by impregnating thestyrene polymer particles with a suitable blowing agent.

To render the polymer particles expandable, the blowing agent isincorporated into the particles, as in U.S. Pat. No. 2,983,692 bysuspending the particles in water in a ratio between 0.3 to 1 and 1.5 to1 (polymer to water) with the aid of suspending agent systems such astricalcium phosphate in combination with a surfactant to promotewetting.

During the impregnation of the particles with blowing agent, otheradditives can also be incorporated, such as internal fast-cool agents,pigments and dyes, stablizers, anti-lump agents, self-extinguishingagents, plasticizers and the like.

The impregnation is conventionally carried out at temperatures rangingfrom about 80° to 150° C. Increasing the temperature makes theimpregnation proceed at a faster rate.

The blowing agents are compounds which are gases or which will producegases on heating. Preferred blowing agents include aliphatichydrocarbons containing from 4-6 carbon atoms in the molecule, such asbutanes, pentanes, hexanes, and the halogenated hydrocarbons which boilat a temperature below the softening point of the polymer. Mixtures ofthese agents may also be used, such as a mixture of 40-60% n-pentane and60-40% trichlorofluoromethane. Usually from 3-20% of blowing agent per100 parts of polymer is incorporated by the impregnation.

The quaternary ammonium salts suitable for the present invention havethe general formula. ##STR1## wherein R may be C₁ -C₁₁ liner or branchedalkyl and mixtures thereof, R' may be oxyethylene, oxypropylene,oxybutylene or mixtures thereof, R" may be C₁ -C₄ linear or branchedalkyl, hydroxylalkyl, or benzyl, x may be an integer from 6 to 20, and Amay be any anion.

The ammonium salts may be prepared by alkoxylation of a C₁ -C₁₁ alcoholwith an alkyleneoxide, amination with dimethylamine, methylamine, ormixtures thereof, and quaternization of the resulting amine, or mixturesof amine. The anion, A, may be especially chloride, bromide, acetate,methyl sulfate or hydroxyl.

The ammonium salts may be coated onto the styrene polymer particles byany suitable method, such as dry blending in a mixer or solution coatingfollowed by evaporation of the solvent. Certain of the quaternary saltsmay be added to the polymer particles during impregnation of theparticles with the blowing agent. In order for impregnation to be auseful method, however, the type of oxyalkylene units used must beselected to minimize water solubility of the salt. A preferred method ofcoating the polymer particles is to fluidize the particles in a fluidbeddryer, and add a solution of the salt to the bed to coat the particleswhile drying off the solvent.

The ammonium salts are added to the polymer particles in amounts between0.1 to 0.4 parts per 100 parts of polymer. Less than 0.1 part of saltdoes not give sufficient improvement in static or antilump propertiesand greater than 0.4 part of salt serves no useful purpose and increasesthe cost of the products.

The invention is further illustrated by the following example whereinparts are by weight unless otherwise indicated.

EXAMPLE I

A series of polystyrene beads with 6-7.5% by weight n-pentane blowingagent impregnated therein and having a particle size of predominantlythrough 16 and on 35 mesh, U.S. Standard Sieve, was fluidized in a LabLine/PRC Hi-Speed Fluid Bed Dryer. The quaternary ammonium salts,usually 5-10% in hexane solvent, were added near the bottom of thefluidized bed with a syringe having a long needle. The end of the needlewas flattened to better disperse the salt solultion. The solution wasadded over several minutes. The particles were coated with 0.2% by wt.,based on polymer particles, of dried salt.

The coated beads were then pre-expanded in a Buccaneer stirred expanderto a density of about 1 pound per cubic foot (pcf). A weighed portion ofpre-expanded beads were then screened through a 4 mesh screen and thepercent lumping determined by weighing the residue of fused pieces onthe screen.

The antistatic properties of the ammonium salts was determined bymolding the samples into 8 inch diameter disks of 2 inch thickness andthen subjecting these to a Static Charge Analyzer, Model 276 fromPrinceton Electro Dynamics, Inc. The samples were charged with an ioncurrent from a corona discharge. Sample disks were aged at the relativehumidities (RH) shown and the peak sample voltage and current charge wasrecorded graphically for 40 seconds. Measurements given in Table Iinclude the initial charge voltage and the % decay in volts for thefirst 5 seconds.

From the table, it can be seen that the trialkyl alkoxylated quaternaryammonium salts of the invention have antilumping properties as well asgreatly enhanced anti-static properties.

                  TABLE I                                                         ______________________________________                                                       1 PCF Foam Properties                                                             Static Test                                                     Quaternary                Volt/   Volt/                                  Run  Salt       %        %     % Decay/                                                                              % Decayl/                              No.  R.sup.(a)                                                                             X.sup.(a)                                                                            Lumping                                                                              Fusion                                                                              22% RH  100% RH                              ______________________________________                                        Con- --      --     23.0   67    1175/0.4.sup.                                                                         1280/3.9.sup.                        trol                                                                          1    Me      10     8.7    53    450/85  320/82                               2    EPAL    8      2.4    --    413/70  343/89                                    610                                                                      3    EPAL    8      6.0    --    380/98  933/76                                    810                                                                      4    EPAL    8      6.0    42    276/83  493/83                                    810                                                                      5    EPAL    8      3.6    42     233/100                                                                               407/100                                  810                                                                      6    EPAL    8      5.1    --    170/92  390/84                                    810                                                                      ______________________________________                                         .sup.(a) Formula for quaternary ammonium salt:                                ##STR2##                                                                      where PO is oxypropylene, Me is methyl, EPAL 610 is a mixture of alcohols     of C.sub.6 -C.sub.10 alkyl sold by Ethyl Corp. and EPAL 810 is a mixture      of alcohols of C.sub.8 -C.sub.10 alkyl sold by Ethyl Corp.               

What is claimed:
 1. Method of making expandable styrene polymer particles which exhibit anti-lumping and anti-static properties comprising coating the surface of expandable styrene polymer paraticles with 0.1 to 0.4 weight percent bases on polymer of a quaternary ammonium salt having general formula ##STR3## Wherein R may be C₁ -C₁₁ linear or branched alkyl and mixtures thereof, R' may be oxyethylene, oxypropylene, oxybutylene or mixtures thereof, R" may be C₁ -C₄ linear or branched alkyl, hydroxylalkyl, or benzyl, x may be a integer from 6 to 20, and A may be any anion.
 2. The method claim 1 wherein said coating is accomplished by dry-blending the expandable polymer particles with the ammonium salt.
 3. The method of claim 1 wherein said coating is accomplished by intimately mixing the expandable polymer particles with a solution of ammonium salt in volatile solvent while allowing the solvent to evaporate off leaving the salt on the surface of the particles.
 4. The method of claim 1 wherein said coating is accomplished by spraying a solution of ammonium salt into a fluidized bed of expandable polymer particles and drying off the solvent.
 5. The method of claim 1 wherein said styrene polymer particles are polystyrene.
 6. The method of claim 1 wherein said styrene polymer particles are a copolymer of styrene with minor amounts of maleic anhydride.
 7. The method of claim 1 wherein said styrene polymer particles are a copolymer of styrene with minor amounts of acrylonitrile.
 8. The method of claim 1 wherein said styrene polymer particles are a copolymer of styrene with minor amounts of methyl acid maleate. 